Precession of the equinoxes
Precession is a slow change in Earth’s axial orientation that shifts the equinoxes relative to the stellar background. Its main cycle is about 26,000 years.
How it is defined
It helps explain the difference between tropical and sidereal reference frames. It does not itself change the definition of the twelve tropical signs.
How to read it
To check the phenomenon, distinguish the calculated position, its reference time and what an observer can see from their location. These related details answer different questions.
A concrete example
When comparing coordinates from distant epochs, check their time reference. A positional difference does not mean ancient observers saw an unrelated sky.
Explore further
Explore the sources and related concepts. When consulting tables or calendars, look for the date, coordinates and reference frame they use.